I found the react-navigation a very good library for not only for navigation between screens but also by providing with other marvelous features.
But, I feel that a few portions of the library need to be made flexible as per developers choice.
Like providing a customizable borderColors for headers.
For reference you check the line number:356 in the below link.
I think if such things can be customizable by passing the the colors through props that would be great.